This invention relates to a novel process for preparing diaphragm-deposited activated cathodes to prevent exfoliation or delamination of the cathode coating from the cathode substrate.
Chlorine and caustic soda are commercially produced by the electrolysis of brine in electrochemical diaphragm cells. Such cells contain, as principle elements, a plurality of anodes, cathodes and diaphragms. The diaphragms used in such cells are deposited directly onto a foraminous cathode and thus form a single unitary structure.
The diaphragms employed in chlor-alkali cells have been traditionally fabricated from asbestos fibers. Asbestos has now been replaced to a large extent with resin-impregnated asbestos materials. In resin-impregnated diaphragms, the resin is added to a slurry of asbestos fibers and is deposited with the asbestos under vacuum onto the cathode. The diaphragm is then sintered to fuse the resin and produce a discontinuous polymer coating which joins adjacent asbestos fibers to form a resin-reinforced diaphragm. Diaphragms of this type having superior dimensional properties are disclosed in U.S. Pat. No. 3,694,281 to Leduc and U.S. Pat. No. 4,410,411 to Fenn et al.
Other, more advanced diaphragms currently being developed are fabricated completely from a synthetic polymer which can also be deposited from a slurry of polymer fibers. Diaphragms of this type are disclosed in U.S. Pat. No. 3,944,477 to Argade. These completely synthetic microporous diaphragms are more dimensionally stable and have superior voltage characteristics then the resin-impregnated diaphragms.
The foraminous cathode employed in chlor-alkali cells has been traditionally fabricated from iron or steel. Steel cathodes exhibit satisfactory voltage characteristics and are also able to withstand the operating environment of the cell which includes exposure to significant amounts of sodium chlorate and sodium hypochlorite. Efforts to improve upon the hydrogen overvoltage of steel cathodes have focused on the use of combinations of metals exhibiting lower hydrogen overvoltage characteristics than iron. These "activated cathodes" employ one or more active metals in the coating to realize low hydrogen overvoltage. Such active metals include transition metals, e.g. iron, cobalt or nickel, as well as noble metals, e.g. platinum, rhodium, ruthenium and iridium. These cathodes may also include a metal which is removable from the coating by leaching or extraction, e.g. in sodium hydroxide, to provide a high surface area. Such metals include, by way of illustration, molybdenum, zinc and aluminum.
The activated coating can be applied directly to a steel or iron substrate by means of electrodeposition, electroplating, thermal decomposition, plasma or thermal spraying, or electroless deposition. See, for instance, U.S. Pat. No. 4,354,915 to Stachurski et al., which discloses activated cathodes having an active coating of electro-deposited nickel, molybdenum and cadmium. Alternatively, a wire mesh having an active coating can be draped over a conventional steel cathode.
The combination of an activated cathode and a resin-impregnated diaphragm or a synthetic diaphragm in a narrow anode-cathode gap would produce the optimal voltage reduction in a cell and consequently offer the most economical performance. Unfortunately, however, such a diaphragm-cathode combination is not presently available on a commercial basis due, in part, to difficulties encountered in manufacturing such a structure. One of these difficulties relates to the exfoliation or delamination of the cathode coating from the substrate following baking or sintering of the diaphragm-deposited activated cathode element. This results in deterioration of the cathode coating and a pronounced increase in the hydrogen overvoltage of the cathode during prolonged usage in a chlor-alkali cell.
It is, therefore, a principal object of this invention to provide an improved process for preparing a diaphragm-deposited cathode which is not subject to delamination or large voltage increases.